The term “terpene” refers to a class of compounds derived from isoprene, which has the molecular formula C5H8. The basic molecular formula for terpenes comprises multiples of C5H8, that is, (C5H8)n where n is the number of linked isoprene units. The isoprene units may be linked together head to head to form linear chains, or they may be arranged to form rings. The terpenes may be classified by the number of terpene units in the molecule. The terpenes include hemiterpenes which comprise a single isoprene unit (e.g., isoprene); monoterpenes which comprise two isoprene units and are represented by the formula C10H16 (e.g., pinene, limonene); sesquiterpenes which comprise three isoprene units and are represented by the formula C15H24 (e.g., farnesene); diterpenes which comprise four isoprene units and are represented by the formula C20H32 (e.g., cembrene, taxadiene); sesterterpenes which comprise five isoprene units and have 25 carbon atoms (e.g., geranylfarnesol); triterpenes which comprise six isoprene units and are represented by the formula C30H48 (e.g., squalene); tetraterpenes which comprise eight terpene units and are represented by the formula C40H64 (e.g., acylic lycopene, monocyclic gamma-carotene, alpha-carotene, beta-carotene); and polyterpenes which comprise long chains containing many isoprene units (e.g., natural rubber which comprises polyisoprene).
The term “farnesene” refers to six closely related chemical compounds, all of which are sesquiterpenes represented by the formula C15H24. These include α-farnesene and β-farnesene, which are isomers, differing by the location of one double bond. α-Farnesene is 3,7,11-trimethyl-1,3,6,10-dodecatetraene and β-farnesene is 7,11-dimethyl-3-methylene-1,6,10-dodecatriene. The alpha form can exist as four stereoisomers that differ about the geometry of two of its three internal double bonds (the stereoisomers of the third internal double bond are identical). The beta form can exist as two stereoisomers about the geometry of its central double bond.
Two of the α-farnesene stereoisomers occur in nature. (E,E)-α-Farnesene is found in the coating of apples, and other fruits, and it is responsible for the characteristic green apple odour. Its oxidation by air gives compounds that are damaging to the fruit. The oxidation products injure cell membranes which eventually causes cell death in the outermost cell layers of the fruit, resulting in a storage disorder known as scald. (Z,E)-α-Farnesene has been isolated from the oil of perilla. Both isomers are also insect semiochemicals; they act as alarm pheremones in termites or food attractants for the apple tree pest, the codling moth. α-Farnesene is also the chief compound contributing to the scent of gardenia.
β-Farnesene has one naturally occurring isomer. The E isomer is a constituent of various essential oils. It is also released by aphids as an alarm pheremone upon death to warn away other aphids. Several plants, including potato species, synthesize this pheromone as a natural insect repellent.